The present invention is directed to disk drives for data processing equipment and in particular to drives that use rotary head positioners.
Disk drives include disks of magnetic material that are rotated rapidly about their axes and have data magnetically written on the magnetic material in concentric circular tracks. Access to the data is provided by read and write heads, which are positioned above the surfaces of the disks by positioning arms. To read data on a single track, a positioner holds the heads in a relatively stationary position as the disk spins beneath it, but the positioner must move the heads to a different radial position with respect to the disk axis if data are to be read or written on a different track.
Disk drives usually use either linear or rotary positioners. A linear positioner moves the heads in a straight line along a disk radius, while rotary positioners pivot about an axis and thus cause the heads to execute arcuate motion that, while being predominantly radial, additionally has a circumferential component. Although commercial products employing both types of positioners are currently available, disk drives that employ rotary positioners enjoy a size and cost advantage. Continued pressure for higher-capacity, lower-cost drives therefore provides a considerable incentive to employ rotary positioners.
Unfortunately, although most currently available drives use common inductive heads for both reading and writing, the need for increased capacity also provides an incentive to employ separate read and write heads, and the use of separate read and write heads presents obstacles to the use of rotary positioners in conventional disk drives. The incentive for separate heads results from a need for increased signal-to-noise ratio. Increases in disk data density are almost inevitably accompanied by reductions in the amount of magnetic material that contributes to the magnetic-field intensity at each data-carrying magnetic transition. Such reductions decrease the strength of the signal that a transition can cause in a read head, but design compromises dictated by the use of the same head for both reading and writing limit the designer's ability to respond to the signal-strength decrease by optimizing the head's reading performance. To obtain the last bit of performance increase in an inductive head, therefore, separation of read and write functions is necessary.
The potential of magnetoresistive heads provides further incentive for head separation. One way that has been proposed to increase read-head performance is to dispense with the widely used inductive read head and employ what is known as a magnetoresistive head. A magnetoresistive head drives current through a film of material, such as NiFe, whose resistivity is a function of the angle between the current density and the magnetic-field intensity. By observing the voltage variations that result from the flow of a known current through a head made of such material, one can observe the magnetic transitions in a desk spinning beneath the head and thus read the data that they encode.
A well-designed magnetoresistive head has the potential for a signal-to-noise ratio significantly higher than that of a comparable inductive head. But such performance can be achieved only if the read head is spaced far enough from its companion write head to prevent the write-head field from causing changes in the magnetoresistive head's magnetization state. Otherwise, the write head could cause the magnetoresistive film to lose the single-domain magnetic state that it is intended to have, and this would subject its output to Barkhausen noise caused by motion of domain boundaries in the film.
But separating the read and write heads causes difficulties that arise from the need to use a read head in conjunction with a write operation. Each circular track on a formatted disk has a plurality of circumferentially spaced data-containing sectors separated by headers, which contain various types of housekeeping information, such as the identity of the track on which the header is found and of the sector that follows the header. Headers typically also include timing bursts, which serve as references for the drive's sampling clock, and they may additionally include fields that have, e.g., bad-block-replacement information and sectored servo information, the latter of which the disk drive uses to locate the centerline of a track.
In order to write in the appropriate place, the drive must first read the header information so that it can ascertain the current write-head location. If the read and write heads are the same, or at least occupy essentially the same space, this presents no problem. However, if the track pitch is very low--i.e., if the number of tracks per unit radial distance is high--then any significant separation between the read head and the write head makes it difficult to use information obtained from the read head to position the write head properly.
This is partly a fabrication problem. An inductive read/write head has roughly the form of a horseshoe magnet. The open end of the horseshoe faces the disk surface with the legs aligned along the track, one leg ahead of the other, so that as the disk spins, each magnetic transition encountered by the head will at some point be located between the legs. In modern disk drives, the head is fabricated in a photolithographic process substantially the same as that employed to make conventional integrated circuits, with one leg of the horseshoe being laid down in one fabrication layer while a spacer and the other leg are laid down in subsequent fabrication layers. The disk-facing surface is provided by slicing through the substrate to reveal a cross section in which moving up in the thickness direction through the substrate means moving circumferentially along the disk track. If a second head is to be spaced farther along the track, therefore, it must be deposited in higher lithographic layers, whose contents are determined in separate masking steps at different substrate depths. Proper alignment of the heads thus becomes a problem of mask alignment. The smaller the track pitch, the more severe the alignment requirement is.
These process considerations apply regardless of the type of head positioner used. But certain other problems, which remain even if these process problems are overcome, are peculiar to the rotary positioner. In moving from track to track, a rotary positioner not only translates the heads but also changes their angular positions. Thus, displacement between heads that has only a desired circumferential component at one positioner angle acquires an undesired radial component when the positioner is oriented at another angle; i.e., if the read and write heads are both exactly on the track centerline at one positioner angle, the centering one of the heads at another positioner angle will cause the other head to be offset. Such an offset can be significant, and even intolerable, if the track pitch is low and the displacement between the heads is great enough to allow proper magnetoresistive-head operation.
One proposal for dealing with this problem is simply to take advantage of the width of the track and orient the positioner so that the heads are offset slightly in opposite directions from the track centerline. But there is clearly a limit to the applicability of this approach; at a certain track pitch, the offset range will exceed the width of the track.
Another approach is to position the read head on the basis of information contained in a header disposed several sectors ahead of the target write location; this gives the positioner control system time to move the positioner radially by the (known) radial offset between the heads. But if the disk drive relies on sectored servo information to center the heads properly, this approach "lengthens" the servo loop, thus making tracking more difficult. And even if only track/sector ID information from the header is important, this approach still extends the latency time and increases the time interval during which the clock must stay in synchronization between timing bursts.
For all these reasons, the trend toward greater data density makes it difficult to retain the advantages of rotary positioners.